Exhaust-gas treatment devices in which use is made of a liquid additive for exhaust-gas purification are generally used in motor vehicles to reduce nitrogen oxide compounds in the exhaust gas of an internal combustion engine. In exhaust-gas treatment devices of this type, the method of selective catalytic reduction (SCR) is implemented. In the SCR method, nitrogen oxide compounds in the exhaust gas of the internal combustion engine are reduced with the aid of ammonia. Ammonia is commonly stored in motor vehicles not directly but rather in the form of a liquid additive which is converted into ammonia outside the exhaust gas (in a reactor provided especially for this purpose outside the exhaust-gas treatment device) or within the exhaust gas (within the exhaust-gas treatment device).
Urea-water solution is particularly commonly used as liquid additive for exhaust-gas purification. A urea-water solution with a urea content of 32.5% is available for example under the trade name AdBlue®.
A problem with the provision of such liquid additive is that the liquid additive can freeze at low temperatures. The described urea-water solution with a urea content of 32.5%, for example, freezes at −11° C. A further problem is that the exhaust-gas treatment device must normally be supplied with liquid additives even at cold temperatures in order to ensure adequate purification of the exhaust gases of an internal combustion engine. For these purposes, it is known to provide heaters in and/or on a tank for storing liquid additive. A further problem is that liquid additives of this type are very aggressive and not every tank, and not every delivery unit, is suitable for the delivery and supply of such liquid additive.